This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application for MICROWAVE OVEN AND VOLTAGE CONTROLLING METHOD THEREOF earlier filed in the Korean Industrial Property Office on Mar. 9, 2001 and there duly assigned Ser. No. 2001-12339.
1. Field of the Invention
The present invention relates in general to a microwave oven and a method for controlling voltage thereof, and more particularly, a microwave oven and a method for controlling voltage thereof, which can lower an anode peak voltage applied to a magnetron during an early non-oscillating period, and remove a surge voltage.
2. Description of the Related Art
Generally, a microwave oven is, as shown in FIG. 4, comprised of a high voltage transformer 55 generating high voltage from external AC (alternating current) power, and a magnetron 51 generating electromagnetic waves.
The secondary part of the high voltage transformer 55 comprises two coils connected in parallel, and having different lengths relative to each other. Where AC power is supplied to the primary coil of the high voltage transformer 55, different voltages are respectively induced in the two coils of the secondary part thereof. In one of the two coils, there is induced a voltage of several volts for heating a filament of the magnetron 51, and in the other one, there is induced an AC pulse voltage of several thousands volts to be supplied to a cathode and an anode of the magnetron 51.
If the voltages induced from the high voltage transformer 55 heat the filament of the magnetron 51 and are supplied to the cathode and the anode of the magnetron 51, the magnetron 51 oscillates, to thereby generate electromagnetic waves.
In order to respectively supply DC (direct current) power to the cathode and the anode of the magnetron 51, in the secondary part of the high voltage transformer 55, there is installed a rectifying circuit. The rectifying circuit includes a rectifying diode 61 and a smoothing capacitor 62 which are connected in parallel with each other. The rectifying diode 61 rectifies the AC pulse voltage from the high voltage transformer 55, and the smoothing capacitor 62 smoothes the AC pulse voltage rectified by the rectifying diode 61.
In this conventional microwave oven, as illustrated in FIG. 5, there are problems in that a circuit element susceptible to voltage is damaged by a surge voltage generated at the beginning of magnetron operation or during the magnetron operation, etc.
Moreover, even if high voltage is respectively supplied to the cathode and the anode, the magnetron 51 does not oscillate until the filament is heated. However, when the magnetron 51 starts to operate, about 8,000 volts of anode peak voltage is supplied to the anode and the cathode until the filament is heated, namely, during a non-oscillating period. Because of the excessive high voltage supplied to the anode and the cathode during the early non-oscillating period, the performance of the magnetron 51 is lowered and the noise of the filament is instantaneously amplified. Further, because an excessive high voltage is supplied to the rectifying diode 61 in an inverse direction, the rectifying diode 61 can be damaged.
In order to solve these problems, in a conventional method, on the power input part of the high voltage transformer 55, there are installed a relay and a resistance which are connected in parallel with each other. At the beginning of supplying electric power, external AC power is supplied to the high voltage transformer 55 through the resistance by turning off the relay for a predetermined time. Then, the resistance can remove the surge voltage.
However, the manufacturing cost is raised because of the relay and the resistance. Also, if the relay is repeatedly turned on and off to operate the microwave oven, the contact of the relay may become defective and the resistance may be damaged. Further, the capacity of the rectifying diode 61 may be increased in order to prevent the rectifying diode 61 from being damaged, but there is a limit to the increase in the capacity.
Accordingly, the present invention has been made keeping in mind the above-described shortcoming and user""s need, and an object of the present invention is to provide a microwave oven and a method for controlling voltage thereof, which can lower an anode peak voltage applied to a magnetron during an early non-oscillating period, and remove a surge voltage.
This and other objects of the present invention may be accomplished by the provision of a microwave oven comprising a magnetron having an anode, a cathode and a filament, and a high voltage transformer having a primary coil and a secondary coil for supplying a high voltage to the magnetron, further comprising a capacitor connected in parallel to the secondary coil of the high voltage transformer, forming a resonance circuit with the secondary coil.
Preferably, the secondary coil is comprised of a first coil part connected to the filament of the magnetron and a second coil part connected to the cathode and the anode of the magnetron; and the capacitor is connected in parallel to the second coil part.
Desirably, the secondary coil of the high voltage transformer is provided with a rectifying circuit, including a smoothing capacitor and a rectifying diode, and the smoothing capacitor is installed between the high voltage transformer and the rectifying circuit.
According to another aspect of the present invention, the above and other objects may be also achieved by the provision of a method for controlling voltage in a microwave oven comprising a magnetron and a high voltage transformer, the method including the steps of converting voltage supplied from an external source into a high voltage through the high voltage transformer, resonating the high voltage, and supplying the high voltage to the magnetron.